In a common network, individual devices perform a communication by using a physical address written in a ROM (Read Only Memory) or the like in advance for each device or an interface board embedded in each device, and a logical address set for each protocol used in an actual communication. Physical addresses are uniformly managed by a manufacturer and provided along with hardware. Logical addresses are set by users for a communication protocol running on a device, and are used to identify a communication partner. For example, in TCP/IP (Transmission Control Protocol/Internet Protocol) communication protocol, a MAC (Media Access Control) address is a physical address, and an IP (Internet Protocol) address is a logical address.
Devices (terminals and repeaters) in a network hold a translation table between a MAC address and an IP address in their cache, and identify a destination of communication by updating information of the cache, for example, with ARP (Address Resolution Protocol). Here, the terminals indicate communication devices such as a server, a PC and the like provided with communication means such as a network interface for which a MAC address and an IP address are set. Moreover, the repeaters are repeaters such as a gateway for connecting two or more networks such as a LAN (Local Area Network) or the like.
Here, if IP address duplication occurs in a network, all terminals having a duplicate IP address are disabled to perform a communication. Therefore, a service is forced to be stopped. The IP address duplication means that the same IP address is assigned to a plurality of different physical addresses.
FIG. 11 is an explanatory view of the IP address duplication in a network system. 20, 21, 22 and 23 illustrated in FIG. 11 are respectively a terminal a, a terminal b, a repeater and a terminal c, which belong to the same network. Each of the terminals has a function of notifying (broadcasting) the other devices (the repeater 22 and the other terminals different from the local terminal) within the network to which the local terminal belongs of association information between an IP address and a MAC address, which is set in the local terminal. The repeater 22 has a function of translating, into a MAC address, an IP address of a destination of a packet, which is included in the packet transmitted from any of the terminals, by using an address translation table, and of transferring the packet to the terminal associated with the MAC address. In the address translation table, association information between an IP address and a MAC address is stored. For example, association information between an IP address IPa and a MAC address MACa of the terminal a illustrated in FIG. 11 is stored in the address translation table. The repeater 22 also has a function of changing association information stored in the address translation table based on association information between an IP address and a MAC address, which is broadcast from each of the terminals.
In the network system illustrated in FIG. 11, if a user erroneously sets the IP address IPa in association with a MAC address MACb of the terminal b, IPa is associated with MACb in the terminal b. In the meantime, IPa is correctly associated with MACa in the address translation table within the above described repeater 22. Therefore, the same IP address IPa is duplicate for the different MAC addresses. Here, the terminal b broadcasts the association information between IPa and MACb to the repeater 22 and the terminal a (see #1 of FIG. 11). The association information between IPa and MACb is information indicating that IPa is associated with MACb. The repeater 22 to which the association information has been broadcast changes the association information between IPa and MACa in the address translation table to the association information between IPa and MACb (see #2 of FIG. 11). As a result, for example, if the terminal c attempts to communicate with the terminal a via the repeater, the terminal c is disabled to communicate with the terminal a since the IP address IPa of the terminal a is associated with MACb in the address translation table within the repeater 22 (see #3 of FIG. 11).
To solve the problem that has been described with reference to FIG. 11 and is caused by the IP address duplication, for example, techniques described below with reference to FIGS. 12 and 13 are proposed.
FIG. 12 illustrates a first conventional technique. Devices denoted with the same reference numerals as those of the devices illustrated in FIG. 11 among devices of FIG. 12 indicate devices similar to those illustrated in FIGS. 11. 30 and 31 illustrated in FIG. 12 respectively indicate terminals a1 and b1. Assume that the IP address and the MAC address of the terminal 1a are IPa and MACa, and the MAC address of the terminal b1 is MACb. Also assume that an association is made between IPa and MACa in the address translation table included in the repeater 22.
With the first conventional technique, if a user sets the IP address IPa for the terminal b1 in association with the MAC address MACb of the terminal b1, the terminal b1 inquires of the other terminal (such as the terminal a1) in the network about a physical address associated with IPa attempted to be set for the local terminal b1 (see #1 of FIG. 12).
Then, the terminal a1 makes, to the terminal b1 in response to the inquiry, a reply such that the physical address associated with IPa is the physical address MACa of the local terminal (see #2 of FIG. 12). The terminal b1 that has received the above reply from the terminal a1 recognizes that IPa is duplicate for the terminal b1 and the terminal a1, and stops broadcasting, to all the devices (such as the repeater 22) belonging to the network (such as a sub-network) to which the terminal b1 belongs, the association information between IPa and MACb (see #3 of FIG. 12). Accordingly, the address translation table within the repeater 22 is not changed, and the terminal c is enabled to communicate with the terminal a1 to be communicated in consequence (see #4 of FIG. 12).
FIG. 13 illustrates a second conventional technique. Devices denoted with the same reference numerals as those of the devices illustrated in FIG. 12 among devices of FIG. 13 indicate devices similar to those illustrated in FIG. 12. 32 illustrated in FIG. 13 indicates a terminal b2. Assume that a MAC address of the terminal b2 is MACb. Also assume that an association is made between IPa and MACa in the address translation table included in the repeater 22.
If a user sets the IP address IPa in association with the MAC address MACb of the terminal b2, IPa is associated with MACb. However, IPa is associated with MACa in the address translation table within the repeater 22. Therefore, IPa is duplicate for the different MAC addresses. With the second conventional technique, the terminal b2 broadcasts the association information between IPa and MACb to the other devices (such as the terminal a1 and the repeater 22) in the network (see #1 of FIG. 13). Then, the repeater 22 changes the association information between IPa and MACa in the address translation table to the association information between IPa and MACb according to the information broadcast from the terminal b2 (see #2 of FIG. 13). Then, the terminal a1 makes, to the terminal b2, a reply such that the physical address associated with IPa is the physical address MACa of the local terminal in response to the broadcast information from the terminal b2 (see #3 of FIG. 13). The terminal b2 that has received the reply from the terminal a1 recognizes that IPa is duplicate for the terminal b2 and the terminal a1, and broadcasts the association information between IPa and MACa indicating whether IPa is associated with MACa to all the devices such as the repeater 22 belonging to a network such as a sub-network to which the terminal b2 belongs (see #4 of FIG. 13). The repeater 22 to which the association information between IPa and MACa has been broadcast from the terminal b2 again changes the address translation table changed in the above #2 (see #5 of FIG. 13), and again associates IPa with MACa. As a result, the terminal c is enabled to communicate with the terminal a1 to be communicated (see #6 of FIG. 13).
An address management method for monitoring communication data that flows in a network and for automatically creating an association between a physical address and a network address of each terminal in an address association table is proposed.    Patent Document 1: Japanese Laid-open Patent Publication No. H11-196106
The first conventional technique described with reference to FIG. 12 assumes that a terminal b1 (hereinafter referred to as a first address duplication resolving terminal) for inquiring of a different device in a network about a physical address associated with an IP address attempted to be set for the local terminal, and for stopping broadcasting association information between the IP address and the physical address of the local terminal to all devices such as the repeater 22 belonging to the network such as a sub-network, to which the local terminal belongs, according to an inquiry result exists in the network. Moreover, the second conventional technique described with reference to FIG. 13 assumes that a terminal b2 (hereinafter referred to as a second address duplication resolving terminal) for broadcasting association information between an IP address set for the local terminal and a physical address of the local terminal to a different device in a network, and for broadcasting correct association information between the IP address and the physical address to all devices (such as the repeater 22) belonging to the network (such as a sub-network), to which the local terminal belongs, according to a result of a reply to the broadcast information from a different terminal (terminal a1 of FIG. 13) for which the IP address is set exists in the network.
However, if logical address duplication that is not intended by a user occurs due to a fault, a trouble or an erroneous setting of a device (such as a repeater) belonging to a network when neither the first nor the second address duplication resolving terminal exists in the network, a terminal that is performing a communication with this logical address is disabled to communicate with a terminal to be communicated.